Look over all cells in a row, column or box. If you find a number which only exist in one cell, this number must be the solution for the cell. Here is a sudoku (which is solved as much as possible with IG and SiSo).

3

1

6

2

8

4

2

6

8

6

5

9

4

7

6

7

9

7

4

8

6

3

7

9

8

7

6

3

4

3

8

5

7

7

5

4

6

8

3

2

1

9

Here is the solution table this far. Notice the number 7 i R1C3. This is the only 7 in the whole R1. This means R1C3 must be 7.